You've made a mistake with the encryption key. It needs to befor an ASCII password, without the "s:" it interprets the key as HEX. These commands will only work for WEP encryption (which is very insecure btw, and can be cracked in just a few minutes with the right software installed). WPA/WPA2 requires something like wpa_supplicant to handle the encryption. The n900 doesn't have wpa_supplicant installed, but since it connects to WPA/WPA2 networks there must be some other mechanism in play, but I haven't worked out what that is yet.

If you want to connect to an already configured network you can follow the guide here.

EDIT: On second inspection; unless "ESSID" is actually part of the wireless network name (the ESSID) it shouldn't be tacked on the end of the network name.